Green Sand Molding
Green sand molding is by far the most diversified molding method used in the present casting processes. The process utilizes a mold made of compressed or compacted moist sand. The term “green” denotes the presence of moisture in the molding sand and indicates that the mold is not baked or dried. The mold material consists of silica sand mixed with a suitable bonding agent “clay” and moisture. To produce the mold, a flask, usually a metal frame, although wood may be used for some processes and types of castings, is placed over the pattern to produce a cavity representing one half of the casting. Compaction is achieved by either jolting or squeezing the mold. The other half of the mold is produced in a like manner and then the two halves are positioned together to form the mold.
If the casting has hollow sections, a core consisting of hardened sand (baked or chemically hardened) is used. Cores are located in pockets formed by projections on the pattern equipment to produce core prints. Should extra support for the cores be required, chaplets or spacers are properly positioned to maintain the required dimension. These will fuse with the molten metal when the casting is poured. This type of molding is the best known of all the sand casting molding methods, as the molds may be poured without further conditioning. This type of molding is also most adaptable to light, bench molding for medium sized castings or for use with production molding machines.
Most metals can be cast by this method. Pattern cost and material cost are relatively low. The processes readily lend themselves to automated systems for high volume work as well as short runs and prototype work. The properties of green sand are adjustable within a wide range, making it possible to use this process with all types of green sand molding equipment and for a majority of alloys poured.
There are practical limits to complexity of design. Machining is often required to achieve the finished product. Dimensional accuracy cannot be controlled as well as with other molding processes, although good standards are possible with quality pattern equipment, modern process controls, and high density molding.
Range of Alloys
|Typical dimensional tolerances, inches||± .010″, ± .030″|
|Relative cost in quantity||Low|
|Relative cost for small number||Lowest|
|Permissible weight of casting||Unlimited|
|Thinnest section castable, inches||1/10″|
|Relative surface finish||Fair to good|
|Relative ease of casting complex design||Fair to good|
|Relative ease of changing design in production||Best|
|Range of alloys that can be cast||Unlimited|
Parting Line Influence
When Parting lines are considered, very close tolerances are difficult to obtain. A parting line absorbs fractions of inches per inch. A foundry is doing well to hold a parting line to 0.015 inch. Additional measurement is added to the casting tolerance.